The context of the present work can best be described as a conventional NMR system. Such a conventional system is described in U.S. Pat. No. 4,742,303 incorporated herein by reference. The invention concerns the use of coupling modulation, manifested by a system of (two or more) spin-half nuclear spins I and S (i.e., an IS spin-half system), during RF (radiofrequency) irradiation of one of the spins, in various types of NMR (nuclear magnetic resonance) experiments.
The terminology used in this disclosure is as commonly used in the NMR literature and examples may be found in the publications cited. The names of the IS spinstates are as used by Sorensen, Eich, Levitt, Bodenhausen and Ernst (Progress in NMR Spectroscopy, 16, 163 (1983)) for the product operator (PO) formalism. However, unlike Sorensen et al we group all the one-spin product operators, Si (i=x, y, or z), together and describe them as in-phase states or magnetization, and we describe all the two-spin product operators, 2SiIj (i=x, y, or z; j=x, y, or z) as antiparallel spinstates or antiparallel magnetization. A second difference, which will be expanded further in this disclosure, is that Sorensen et al describe spinstates as observable only if they provide detectable magnetization during signal acquisition. In this disclosure we are concerned with the actual measurable state of an IS system at any time during an RF pulse or sequence of pulses. We consider a state to be measurable if maximum detectable signal can be obtained after a hard on-resonance pulse on the S spins and/or a hard on-resonance pulse on the I spins. Thus the orthogonal PO states are convenient descriptors of the total state because the total state can be resolved into a linear combination of the PO states, and each PO state is measurable after a hard on-resonance 90xc2x0 pulse on the S and/or I spins.
For an IS J-coupled system at the beginning of a 1D (one-dimensional) decoupled acquisition, it has been considered impossible to produce detectable signal from pure antiparallel spinstates, 2SyIj (j=x, y, or z), because it has been assumed that the antiparallel components self cancel (e.g. see Levitt, Bodenhausen and Ernst, Journal of Magnetic Resonance 53, 443 (1983)). However, we find that a decoupler field of constant amplitude and frequency, applied to the I spins while observing the S spins, establishes an additional channel for communication between I and S which produces, via J-mediated effects, a coupled time evolution yielding directly detectable in-phase transverse magnetization, Sx. This mechanism provides an initial basis for the embodiments of the invention. The S-spin signal produced whilst continuously irradiating the I spins may appear as sideband or centerband intensity depending on the initial antiparallel spinstates. Alternatively, discrete I-spin irradiation (RF pulses) may enable 100% transformation between different antiparallel and in-phase spinstates. An exact QM (quantum mechanical) analysis of this behaviour, generalized for all possible intial conditions, provides a means to calculate and design new selective RF pulses, and sequences of RF pulses. Several fast and efficient methods for characterizing the Insensitive I-spin channel of a spectrometer by observing the sideband signals with the Sensitive S-spin channel are described. These various methods of design and display comprise embodiments of the invention.
The detailed QM study of CW (continuous wave) decoupling of I spins in an IS spin system, by Anderson and Freeman (Journal of Chemical Physics 37, 85 (1962)), and other pioneering work noted by these authors, provided an analytical solution for the amplitude and frequency of the decoupled Sx signal starting with pure in-phase magnetization, Sx, of the observed nucleus. However, the Sx spinstate is just one of 15 possible IS product operator spinstates that can arise from irradiation of either the I or S spins. In general, any of the 15 PO spinstates can be produced beginning with any single PO spinstate so there are 15xc3x9715=225 analytical equations that completely describe an IS spin system subject to RF irradiation. The theoretical basis of the invention required the derivation of the other 224 equations, and all the embodiments of the invention for an IS spin system use the entire set or subsets of these 225 equations.
In a first general embodiment of the invention, a method is obtained to allow the rapid calculation of and determination of the result of a complex series of RF pulses and time delays (a pulse sequence) and thus can be used to design RF pulse sequences or the equipment that delivers RF pulse sequences or to determine the exact result of a pulse sequence. For example, the invention can be used to display the mechanism of RF pulse sequences for each time increment throughout the sequence for the purposes of education, software/hardware debugging, or the discovery of new pulse sequences.
In a second specific embodiment of the invention, continuous RF irradiation generates sideband signals whose frequency depends on the amplitude of the irradiation and thus the power and homogeneity of the radiofrequency source. This information may be used to assess or improve the design of the NMR probe that produces and detects the radiofrequency energy.
In a third general embodiment of the invention, RF pulses of low power interconvert orthogonal PO (product operator) spinstates of the IS system. This interconversion is frequency selective and thus may be used to discriminate between spins in differing chemical environments that consequently resonate at different frequencies.
Whilst all the examples and illustrations of the invention presented in this specification are in the context of J (or scalar) coupling between the two types of nuclear spin, the theoretical basis of the invention is also valid for dipolar coupling, thus permitting the invention to be applied to two or more dipolar-coupled spins in solids or partially-aligned chemical systems.
For simplicity all the examples in this specification are in terms of two single spins I and S, but the invention also applies to multiple I and S spins as in an ImSn spin system. Provided the RF irradiation is applied to only one type of spin at any one time, the invention is equally applicable to homonuclei or heteronuclei. In this context, the distinguishing feature of different types of homonuclei is that they must resonate at different NMR frequencies. Since the invention is applicable to RF irradiation of only one type of spin at any one time, the behaviour of more than two types of coupled spins can be determined by sequential application of the invention to each pair of spin types, I and S.
Although common aspects of NMR spectroscopy such as pulsed field gradients and nuclear spin relaxation do not form a part of the invention, such aspects may, nevertheless, be included within calculations of RF pulse sequences as expressed in the first general embodiment of the invention. The new art comprises a means of rapidly calculating the effect of RF pulses inclusive of coupling modulation, and thus a new fundamental means of analysing NMR experiments. Other common attributes such as gradients and relaxation can be added to this fundamental approach using conventional protocols.